Data_Representation.ppt 1515KB Jun 23 2011 12:05:58 PM

Chapter 2
Computer Hardware
Section A: Data Representation
and Digital Electronics

Computer Concepts 7th Edition
Parsons/Oja

Data Representation: How do computers
represent data digitally?
• Data is defined as the symbols that represent things,
people, events and ideas
• Computers store data in digital format as a series of
1s and 0s (known as binary code)
– Each 1 and 0 is called a bit
– Eight bits is called a byte

• The term bit comes from “binary digit”
• Bytes are used to represent one character – a letter,
number, or punctuation mark
– For example, the letter H is represented in binary code as

01001000
– An exclamation point (!) is 001000001

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Data Representation: How do computers
represent data digitally?
• Digital data is made up of discrete numbers,
with each bit being either a 1 or a 0 – it’s either
on or off, nowhere in between
• Analog data is made up of a continuous wave
of information, with varying degrees in between
• For example:
– A digital clock changes it’s digital display
once every minute to show the time
– An analog clock is continually moving it’s
second, minute and hour hands to show
the time

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Data Representation: How do computers
represent data digitally?

• Another example is a light fixture
– A standard light switch is similar to digital
• It is either on or off – 1 or 0

– A dimmer light switch is
similar to analog
• It’s rotating dial can be
turned to many different
positions to make the light
varying degrees of bright
or dim

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Data Representation: How do computers
represent data digitally?
• Data representation makes it possible to
convert letters, sounds, and images into
electrical signals
• Digital electronics makes it possible for
computer to manipulate simple “on” and “off”
signals to perform complex tasks
– A computer’s circuits have only two states: on
and off
– A binary 1 represents “on”
– A binary 0 represents “off”

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How can a computer represent
numbers?
• Unlike the decimal system (base 10), the binary
number system (base 2) uses only two digits:
0 and 1
• The following table lists
some decimal numbers
and their binary
equivalent:

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How can a computer represent
words and letters using bits?
• Character data is composed of letters, symbols,
and numbers that will not be used in arithmetic
operations

– Numeric data is used in arithmetic calculations, and is
encoded differently

• ASCII (American Standard Code for Information
Interchange) requires only 7 bits for each character
• Extended ASCII uses 8 bits for each character.
Used in most personal computers
– See the code on the next slide

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How can a computer represent
words and letters using bits?

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How can a computer represent
words and letters using bits?
• EBCDIC (Extended Binary-Coded
Decimal Interchange Code) is an
alternative 8-bit used by older IBM
systems
• Unicode uses 16 bits and provides
codes for 65,000 characters – a bonus
for representing alphabets of multiple
languages
– Used for foreign language support
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How does a computer convert
sounds and pictures into codes?
• Sounds and pictures must be
transformed into a format the computer
can understand

• A computer must digitize colors, notes,
and instrument sounds into 1s and 0s
• For example, a red dot on your screen
might be represented by 1100, a green
dot by 1101
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How does a computer store all
these codes?
• Data is stored on a computer in a file
– Data files might contain the text of a document, the
numbers for a calculation, the contents of a web page, or
the notes of a music clip as binary code
– Executable files contain the programs or instructions that
tell the computer how to perform a specific task. For
example, how to display and print text

• Data files have a file header which tells the

computer how the binary code is used to represent
the data.
– The header tells the computer if the binary code represents
a music file, a graphic, a text document, etc.
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Quantifying Bits and bytes: How
can I tell the difference between
bits and bytes?
• A bit is one binary digit (b)
– Eg. 0

• A byte is 8 bits (B)
– Eg. 0010 0100

• A nibble is 4 bits
– Eg. 0011

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Quantifying Bits and bytes: How
can I tell the difference between
bits and bytes?
• Prefixes
– Kilo- means a 1000
– Mega- means million
– Giga- means billion

• Kilobit (Kb) is approx. 1,000 bits (1,024)
• Kilobyte (KB) is approx. 1,000 bytes (1,024)
• Megabyte (MB) is approx. 1,000,000 bytes
(1,048,576)
• Gigabyte (GB) is approx. 1,000,000,000 bytes
(1,073,741,824)
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Self Quiz Questions
1.

2.
3.
4.
5.

A(n) _______ device works with discrete
numbers, whereas a(n) _______ device works
with continuous data.
The _______ number system represents numeric
data as a series of 0s and 1s.
Most personal computers use the _______ code
to represent character data.
100 Mb is larger than 100 MB. True or false?
A prefix that means a million bytes is _______.

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Self Quiz Answers
1.

2.
3.
4.
5.

A(n) digital device works with discrete numbers,
whereas a(n) analog device works with
continuous data.
The binary number system represents numeric
data as a series of 0s and 1s.
Most personal computers use the extended
ASCII code to represent character data.
100 Mb is larger than 100 MB. False
A prefix that means a million bytes is Mega .

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